This invention relates to process for preparing metal nitrates. More particularly, the invention relates to a process for preparing metal nitrates from the corresponding metal wherein the metal is silver, cadmium, bismuth or one of the metals of atomic numbers 24-30.
Metal catalysts, such as nickel and cobalt catalysts, are commercially prepared from the corresponding metal nitrate. Aqueous metal nitrate solutions have been prepared in the art by dissolving the metal in nitric acid. When the metal is reacted with nitric acid, considerable amounts of ammonium nitrate are formed as a contaminant as a result of uncontrollable side reactions. When metal nitrate solutions containing significant amounts of ammonium nitrate are utilized to form metal catalysts, the ammonium nitrate must be separated prior to use of the metal nitrate by means of complex and expensive purification processes, or the contaminated metal nitrate is utilized in the catalyst preparation requiring the discharge of large amounts of ammonia in the waste system.
One method for avoiding the formation of ammonium nitrate involves utilization of a metal oxide or metal carbonate as the starting material rather than the metal. Reaction of nitric acid with metal oxides or metal carbonates produces metal nitrate solutions which are free of ammonium nitrate. However, the starting materials, namely, metal oxides and metal carbonates are generally substantially more expensive than the free metal, and also generally of lower purity than a metal obtained for example, by electrolytic deposition or by decomposition of a metal carbonyl.
When nitric acid is reacted with a metal such as nickel, at least five competing reactions can occur:
xe2x80x83Ni+4HNO3xe2x86x92Ni(NO3)2+2NO2+2H2Oxe2x80x83xe2x80x83(1)
3Ni+8HNO3xe2x86x923Ni(NO3)2+2NO+4H2Oxe2x80x83xe2x80x83(2)
4Ni+1OHNO3xe2x86x924Ni(NO3)2+N2O+5H2Oxe2x80x83xe2x80x83(3)
4Ni+1OHNO3xe2x86x924Ni(NO3)2+NH4NO3+3H2Oxe2x80x83xe2x80x83(4)
5Ni+12HNO3xe2x86x925Ni(NO3)2+N2+6H2Oxe2x80x83xe2x80x83(5)
As can be seen from the above equations, in addition to the desired nickel nitrate, ammonium nitrate and various nitrogen oxides are formed in the reaction. However, the presence of the nitrogen oxides is not as significant a problem as the presence of ammonium nitrate. Thus, it is desirous to develop a procedure for preparing metal nitrates from the metal and nitric acid which suppresses or significantly reduces the extent of reaction 4.
A process for preparing nickel nitrate from nickel and nitric acid is described in the Russian Journal of Inorganic Chemistry 1959, 11, 1122. The authors recommend adding 30% hydrogen peroxide to the reaction mixture. Based upon the ammonium nitrate, at least stoichiometric amounts of hydrogen peroxide are required, and significant amounts of hydrogen peroxide are consumed under the reaction conditions due to decomposition.
It has also been suggested in Dorofeeva et al, Khim Prom-st. (Moscow) 1974, (8), 603-6, Chem. Abs. 81, 172286 m (1974) that the formation of ammonium nitrate can be suppressed during the reaction of nickel with nitric acid by adding metallic copper to the nickel to be dissolved or by using copper-containing nickel for the reaction with nitric acid. A disadvantage of this process, however, is that the nickel nitrate solution obtained is contaminated with copper nitrate which must be removed using additional purification steps.
In USSR Patent 126,482, Mar. 1, 1960, a procedure for preparing nickel nitrate is described which involves continuously dissolving metallic nickel in nitric acid containing 700 to 1000 g/l of nickel nitrate hexahydrate. It is suggested that this procedure prevents the formation of undesirable ammonium nitrate and the corrosive NO and NO2. The process is automatically regulated by the residual concentration of nitric acid in the final solution which must be no more than 1 gram per liter.
A process for the preparation of aqueous metal nitrate solutions by dissolving the metal and nitric acid is described in U.S. Pat. No. 5,039,502 (Horn et al). The formation of ammonium nitrate is reported to be reduced by adding nitrous acid or a substance which forms nitrous acid to the reaction medium. Generally, the reaction is conducted at elevated pressure, and pressures of from 0.1 to 10 MPa are described as being useful, particularly to prevent decomposition of the nitrous acid. This patent describes the preparation of the nitrates of the elements having the atomic numbers 24 to 28 which are iron, chromium, manganese, cobalt and nickel.
U.S. Pat. No. 4,808,393 describes the process for the manufacture of aqueous ferric nitrate solution. The process is reported to overcome the passivation problem associated with the reaction of nitric acid and iron. The process involves continuously gravity-flowing nitric acid through a bed of pieces of metallic iron at a critical reaction temperature less than the ferric nitrate-to-ferric oxide decomposition temperature, and then recycling the effluent through the bed of iron pieces. The effluent is increasingly enriched in ferric nitrate and depleted in unreacted nitric acid by the recycling.
U.S. Pat. No. 2,581,519 also describes the manufacture of metal nitrate by the reaction of nitric acid with a metal such as silver or bismuth. The reaction takes place in an atmosphere of oxygen or a gas consisting mainly of oxygen which replaces the oxygen used in converting the evolved nitrogen oxides directly to nitric acid. Oxygen is introduced into the reaction vessel to expel all of the air from the vessel.
The invention relates to a process for preparing metal nitrates from the corresponding metal wherein the metal is selected from silver, cadmium, bismuth and the metals of atomic number 24-30. The process comprises
(A) providing a reactor containing
(a) the metal,
(b) nitric acid, and
(c) water
xe2x80x83wherein the initial concentration of the nitric acid in the water in the reactor is from about 50% to about 80% by weight, and the reactor is free of
(1) added fuming nitric acid,
(2) added chromium compounds when the metal is iron, and
(3) added oxygen, and
xe2x80x83when the metal is nickel, the reactor contains less than 500 g/l of any added nickel nitrate hexahydrate;
(B) maintaining the temperature within the reactor at a temperature to facilitate the formation of the metal nitrate and to maintain the produced metal nitrate in the molten state;
(C) maintaining the pressure within the reactor at between atmospheric pressure up to about 100 psig; and
(D) recovering the metal nitrate from the reactor, provided that when the metal is iron, any recovered iron nitrate is not recycled.
The process of the present Invention results in the formation of metal nitrates and more particularly aqueous solutions of metal nitrates containing reduced amounts of ammonium nitrate.
The metal nitrates which can be prepared by the process of the present invention include silver nitrate, cadmium nitrate, bismuth nitrate, and the nitrates of the metals of atomic numbers 24-30 which are chromium nitrate, manganese nitrate, iron nitrate, cobalt nitrate, nickel nitrate, copper nitrate and zinc nitrate. In one embodiment of the present invention, the metal nitrates are nickel nitrate and cobalt nitrate. It is also possible to utilize mixtures of the above-identified metals. However, if it is desired to prepare a mixture of metal nitrate solutions, it is generally easier to dissolve the metal separately in nitric acid and subsequently to mix the finished metal nitrate solutions in the desired ratio. The metal which is to be reacted with the nitric acid can be in various physical forms such as in powder form or in larger pieces such as cathode plates and broken cathode plates. When the metal is too finely divided, there is the disadvantage of the formation of undesired metal dust, and when the reaction is conducted in the vertical reactor, undesired metal packing may result. Thus, the use of larger metal particles such as microspheres, granules, electrode cuttings, brochets or crowns is preferred.
The nitric acid utilized in the process of the invention generally has a concentration of at least 50% up to about 80% by weight, and more often, a concentration of nitric acid of at least about 60% up to about 80% by weight. Concentrations of from about 65% to about 70% by weight of nitric acid have been found to be particularly useful.
The metal can be reacted with the nitric acid in either a batch process or in a continuous process. In one embodiment of a batch process, the metal to be dissolved is placed in a reactor, and the nitric acid is added gradually, such as by dripping or spraying, at a rate to control the rate of the reaction. Alternatively, the metal and water can be placed in the reactor, and concentrated nitric acid is added gradually to the reactor. It also is possible to charge the metal, nitric acid and water to the reactor all at once although control of the reaction may be more difficult with some metals.
In one embodiment, the process of the present invention is a continuous process wherein the metal is contained in a reactor such as a vertical reactor having a top opening and a bottom opening wherein the reactor contains a bed of pieces of the metal. The concentrated nitric acid is added, for example, by dripping or spraying into the top of the reactor whereby the nitric acid flows through the bed of metal pieces and the contact time between the nitric acid and the metal pieces is sufficient to form the metal nitrate. The reactor is maintained at a temperature which is sufficient to promote the reaction and maintain the metal nitrate in the molten state so that it will not solidify as it is formed in the reactor. The desired metal nitrate is then recovered from the bottom opening of the reactor.
The reaction, as noted above, generally is carried out at a temperature which is sufficient to maintain the produced metal nitrate in the molten state until the molten metal nitrate can be diluted with water. Thus, the reaction temperature will be determined at least in part on the temperature at which the metal nitrate tends to solidify. Generally, temperatures are from about 50xc2x0 C. to about 150xc2x0 C., and temperatures of from about 60xc2x0 C. to about 90xc2x0 C. also are useful. The reaction between the metal and nitric acid may be conducted at atmospheric pressure, although the reaction can be conducted at slight pressures such as from atmospheric pressure up to about 100 psig. In other embodiments, the reaction can be conducted at pressures of from atmospheric pressure up to about 25 psig or even 50 psig. In some instances, conducting the reaction at these slight pressures results in reduced production of ammonium nitrate as an impurity. The pressure generally is generated by the formation of reaction gases.
When the process of the invention is to be conducted in a continuous or semi-continuous manner, vertical reactors, and more particularly, vertical tubular reactors are desirable. The design of the reactor is not critical, and a variety of vertical reactors can be utilized provided that the reactor allows sufficient contact between the nitric acid and the metal to produce the desired metal nitrate within the reactor. In one embodiment, the height of the reactor may range from about 3 feet to about 30 feet, and the diameter of the reactors may vary from between about 3 inches to about 12 inches or more. The height of the bed of metal in the reactors may range from about 2 to about 20 feet, and in other embodiments may range from about 10 to about 12 feet.
In one embodiment, the molten metal nitrate which is formed in the reactor is recovered and solidified by cooling. The cooling process can be controlled to produce the solid product in various forms such as flakes, powders, pellets, etc.
In another embodiment, the molten metal nitrate which is formed in the reactor (either in a batch, continuous or semi-continuous process) is diluted with water before the molten metal nitrate is cooled to a solid. In a batch process, water may be added to the reactor containing the molten metal nitrate or the metal nitrate may be added to a container containing water. When the metal nitrate is formed in a vertical reactor, water may be added to the molten nitrate in the reactor as the nitrate exits from the bed of metal, or water can be added to the molten nitrate upon removal from the reactor and prior to cooling. The amount of water added to the molten metal nitrate is generally an amount sufficient to provide a solution wherein the metal nitrate does not crystallize when the solution is cooled to ambient temperature. In one embodiment, sufficient water is added to provide a metal nitrate solution containing from about 10 to about 20% by weight of metal, and more often, from about 11 to about 15% by weight of metal.
In addition to containing the desired concentration of metal nitrate, the aqueous metal nitrate solutions obtained by the process of the invention will contain only small amounts of unreacted nitric acid. In one embodiment, the aqueous metal nitrate solutions will contain less than about 10% or less than about 5% by weight of nitric acid, and in other embodiments, the aqueous metal nitrate solutions may contain less than about 1% or 2% by weight of nitric acid. In yet another embodiment, the aqueous metal nitrate solutions obtained by the process of this invention may contain less than about 0.01% by weight of unreacted nitric acid.
The metal nitrate solutions prepared in accordance with the process of the present invention, particularly the continuous processes described herein, contain reduced amounts of ammonium nitrate. Metal nitrate solutions having an average adjusted ammonia concentration of less than 1000 ppm of ammonia, or less than 500 ppm, or even less than 100 ppm or 50 ppm of ammonia can be obtained. The average adjusted ammonia concentration is calculated as if the product solution contained 15% w metal as metal nitrate and to account for added water. The solution from the process can contain up to about 20% w or more of metal before dilution to prevent solidification. After this dilution to about 15% w metal, the residual concentration of ammonia can be 50 ppm or lower. Ammonia nitrate production, in some embodiments, also is reduced when the reaction is conducted in the presence of air and in the absence of any added oxygen. In some instances, metal nitrates with reduced ammonium nitrate are obtained when the pressure in the reactor is above atmospheric such as, for example, at 25 or 50 psig.
In one embodiment, the process of the present invention relates to the preparation of metal nitrates from the corresponding metal wherein the metal is selected from silver, cadmium, bismuth or the metals of atomic numbers 24-30, wherein the process comprises
(A) providing a reactor containing
(a) the metal,
(b) nitric acid, and
(c) water
xe2x80x83wherein the initial concentration of the nitric acid in the water in the reactor is from about 50% to about 80% by weight, and the reactor is free of
(1) added fuming nitric acid,
(2) added chromium compounds when the metal is iron, and
(3) added oxygen, and
xe2x80x83when the metal is nickel, the reactor contains less than 500 g/l of any added nickel nitrate hexahydrate;
(B) maintaining the temperature within the reactor at a temperature to facilitate the formation of the metal nitrate and to maintain the produced metal nitrate in the molten state;
(C) maintaining the pressure within the reactor at between atmospheric pressure up to about 100 psig; and
(D) recovering the metal nitrate from the reactor, provided that when the metal is iron, any recovered iron nitrate is not recycled.
In one embodiment of the above process, the reactor is free of any added nitrous acid or free of material which forms nitrous acid such as fuming nitric acid. In the process of the invention, the reactor is essentially free of any added oxygen which includes air as well as pure oxygen. When the metal is nickel or cobalt, no air or oxygen is added, and in one embodiment, any air that is present at the beginning of the reaction is flushed out of the reactor. Alternatively no air or oxygen is added, and any air or oxygen present at the beginning of the process is quickly consumed (burned). In another embodiment, the metal utilized in the above process is nickel or cobalt, and when the metal is nickel, the reactor contains less than 500 g/l or even less than 100 g/l of any added nickel nitrate hexahydrate. In one embodiment, when the metal is nickel the reactor is essentially free of any added nickel nitrate hexahydrate. The phrase xe2x80x9cinitial concentrationxe2x80x9d of the nitric acid as used in the written description and appended claims refers to, in the case of batch processes, to the overall concentration of nitric acid in the water in the reactor at the beginning of the reaction. Thus, for example, if separate additions of two or more different concentrations of nitric acid are added to a reactor, xe2x80x9cinitial concentrationxe2x80x9d is the overall concentration achieved by the two or more additions. In a continuous process, the phrase refers to the overall concentration of nitric acid in water added to the reactor on a continuous or semi continuous manner. The term xe2x80x9cfree ofxe2x80x9d as used in the written description and appended claims is not intended to exclude minor amounts of the stated material which may be present in amounts of less than 1% or even less than 0.1%, such as, for example materials that are present as impurities.
In another embodiment, the process of the present invention relates to the preparation of metal nitrates from the corresponding metal wherein the metal is selected from nickel and cobalt, and the process comprises the steps of
(A) providing a reactor containing
(a) the metal,
(b) nitric acid, and
(c) water
xe2x80x83wherein the initial concentration of the nitric acid in the water in the reactor is from about 50% to about 80% by weight, the reactor is free of any added fuming nitric acid, and when the metal is nickel the reactor contains less than 500 g/l, or less than 100 g/l of any added nickel nitrate hexahydrate;
(B) maintaining the temperature within the reactor at a temperature to facilitate the formation of the metal nitrate and to maintain the produced metal nitrate in the molten state;
(C) maintaining the pressure within the reactor at between atmospheric pressure up to about 100 psig; and
(D) recovering the molten metal nitrate from the reactor.
The molten metal nitrate that is recovered may be allowed to solidify as described above, or the molten metal nitrate can be diluted with water to form a solution and avoid solidification.
When the above process is a batch process, the reactor initially may be provided with metal, and the nitric acid and water may be added as an aqueous nitric acid solution to the metal at a rate which is sufficient to control the rate of the reaction. The temperature of the reaction also can be controlled by the rate of addition of the nitric acid. When the metal is nickel, the reactor is free of any added nickel nitrate hexahydrate.
In yet another embodiment, the process of the present invention for preparing metal nitrates from the corresponding metal is a continuous process which comprises
(A) providing a vertical reactor having a top opening and a bottom opening wherein the reactor contains a bed of pieces of the metal;
(B) feeding an aqueous nitric acid solution which contains less than 500 g/l, more often less than 100 g/l of added nickel nitrate hexahydrate when the metal is nickel, and which solution contains at least 50% nitric acid but is free of fuming nitric acid into the top opening of the reactor whereby the nitric acid flows through the bed of metal pieces, and the contact time between the nitric acid and the metal pieces is sufficient to form the metal nitrate;
(C) maintaining the temperature within the reactor at a temperature sufficient to maintain the metal nitrate in the molten state;
(D) maintaining the pressure within the reactor at between about atmospheric pressure up to about 100 psig; and
(E) recovering the metal nitrate from the bottom opening of the reactor, provided that when the metal is iron, any recovered iron nitrate is not recycled.
As mentioned above, the reactor may be a vertical tubular reactor which may range in height from about 3 feet to about 30 feet, and the diameter of the reactors may vary from between about 3 inches to about 12 inches or more. The height of the bed of metal in the reactor is generally less than the height of the reactor, and thus, in one embodiment, the height of the bed of metal in the reactor may range from about 2 to about 20 feet, and in other embodiments, from about 12 to 12 feet.
In yet another embodiment, the process of the present invention is a continuous process for preparing metal nitrates from the corresponding metal wherein the metal is nickel or cobalt, and the process comprises
(A) providing a vertical reactor having a top opening and a bottom opening wherein the reactor contains a bed of pieces of the metal;
(B) feeding an aqueous nitric acid solution which is free of nickel nitrate hexahydrate when the metal is nickel and containing from about 50% to about 80% nitric acid into the top opening of the reactor whereby the nitric acid flows through the bed of metal pieces and the contact time between the nitric acid and the metal pieces is sufficient to form the metal nitrate;
(C) maintaining the temperature within the reactor at a temperature sufficient to maintain the metal nitrate in the molten state;
(D) maintaining the pressure within the reactor at between about atmospheric pressure up to about 50 psig; and
(E) recovering the metal nitrate from the bottom opening of the reactor.
As mentioned previously, in one embodiment, the nitric acid which is fed to the reactor is free of nitrous acid or materials that can form nitrous acid.